In existing seat belt apparatuses mounted on a seat of a vehicle, such as an automobile, an occupant is restrained by a seat belt in the event of an emergency in which a deceleration greater than a normal deceleration is applied to the vehicle, such as in the event of collision (hereinafter simply referred to “in the event of an emergency”). Examples of such seat belt apparatuses include a three-point seat belt apparatus, which has been widely known and used. In the widely known three-point seat belt apparatuses, a seat belt withdrawn from a seat belt retractor is guided by a belt guide anchor, and a top end of the seat belt is secured to the body structure of the vehicle. The seat belt guided by the belt guide anchor slidably supports a tongue. In such a case, the seat belt is slidably inserted into an elongate seat belt insertion hole of the tongue. Thereafter, by engaging the tongue with the buckle secured to the body structure of the vehicle, the seat belt can be worn by the occupant.
When a seat belt is worn by an occupant, the seat belt located between the belt guide anchor and the tongue functions as a shoulder belt that passes across the shoulder and chest of the occupant. In addition, the seat belt located between the end secured to the body structure of the vehicle and the tongue functions as a lap belt that passes across the lap (the waist) of the occupant. In the event of an emergency, a lock mechanism of the seat belt retractor operates and, thus, withdrawal of the seat belt is prevented. In this manner, the shoulder belt restrains the shoulder and chest of the occupant, and the lap belt restrains the lap of the occupant.
In general, in such seat belt apparatuses, even when the lock mechanism of the seat belt retractor operates in the event of an emergency and prevents withdrawal of the seat belt, the lap belt slightly stretches due to the force of inertia of the occupant. Accordingly, it is difficult for the lap belt to effectively restrain the occupant.
Therefore, the following tongue has been developed. That is, in a normal operation performed by a seat belt, the friction between the seat belt and a seat belt sliding surface of the tongue is reduced and, thus, the seat belt smoothly slides on the tongue. In contrast, in the event of an emergency, the friction between the seat belt and the seat belt sliding surface is increased and, thus, the seat belt negligibly moves toward the lap belt (refer to, for example, PTL 1).
According a tongue described in PTL 1, a seat belt sliding surface of a seat belt insertion hole of the tongue has a plurality of axis-direction grooves each substantially linearly extending in the length direction of the seat belt insertion hole in series and a plurality of circumferential-direction grooves each extending in a direction substantially perpendicular to the length direction of the seat belt insertion hole in series. In such a case, the axis-direction grooves and the circumferential-direction grooves are formed over the entire area of the seat belt sliding surface of the elongate seat belt insertion hole in the length direction. In a normal operation of the seat belt, the seat belt virtually slides on the top end of a convex portion formed by the axis-direction groove and circumferential-direction groove. In this manner, friction between the seat belt and the seat belt sliding surface of the tongue is reduced. As a result, in a normal operation of the seat belt, the seat belt smoothly slides on the tongue. In contrast, in the event of an emergency, two portions of the seat belt located on either side of the tongue are pulled due to the force of inertia of the occupant. Thus, the tension force of the seat belt increases, and the seat belt sinks into the axis-direction grooves and the circumferential-direction grooves due to the increased tension force. Accordingly, friction between the seat belt and the seat belt sliding surface increases. As a result, in the event of an emergency, the seat belt negligibly moves toward the lap belt and, thus, stretch of the lap belt is prevented. By preventing stretch of the lap belt in this manner in the event of an emergency, effective restraint of the occupant using the lap belt can be achieved.
In addition, according to PTL 1, the seat belt sliding surface includes a plurality of linear protrusions each substantially linearly extending in the length direction of the seat belt insertion hole in series. The linear protrusions protrude from the seat belt sliding surface at predetermined intervals in the circumferential direction. By using such linear protrusion, friction between the seat belt and the seat belt sliding surface occurring in the event of an emergency can be changed from that in the normal operation of the seat belt. In such a case, in the normal operation of the seat belt, the seat belt slides on only the surfaces of the linear protrusions and, thus, friction between the seat belt and the seat belt sliding surface is reduced. In contrast, in the event of an emergency, the linear protrusions lean toward the lap belt due to an increased tension force of the seat belt. Accordingly, the seat belt also slides on the surfaces of other portions of the seat belt sliding surface. As a result, the friction between the seat belt and the seat belt sliding surface can be increased.
Furthermore, according to PTL 1, a seat belt sliding surface has a plurality of resin members protruding therefrom. By using such resin members, friction between the seat belt and the seat belt sliding surface occurring in the event of an emergency can be changed from that in the normal operation of the seat belt. In such a case, in the normal operation of the seat belt, the seat belt slides on only the surfaces of the resin members and, thus, friction between the seat belt and the seat belt sliding surface is reduced. In contrast, in the event of an emergency, the resin members get crushed or cut away by an increased tension force of the seat belt. Accordingly, the seat belt also slides on the surfaces of other portions of the seat belt sliding surface. As a result, the friction between the seat belt and the seat belt sliding surface can be increased.